The present invention relates to an electro-hydraulic servo actuator with function for adjusting rigidity which actuator can be used in an oil hydraulic circuit for hydraulic equipment.
A conventionally used electro-hydraulic servo actuator comprises an electro-hydraulic transducing means which transduces an electric signal into a hydraulic signal, an actuator which is operated by said electro-hydraulic transducing means, a position detector which detects a position of an output member of said actuator to emit a positional electric signal, an adder which compares said positional signal from said position detector with a command signal to emit a deviation signal, and an amplifier which amplifies said deviation signal from said adder and applies it to said electro-hydraulic transducing means.
FIG. 5 shows the relationship between a servo error and output F under a normal condition wherein such a conventional electro-hydraulic servo actuator is steady under an external force F.sub.0.
Because of the influence of the external force, there is a difference between a desired value of a servo system and an actual position of an actuator rod, which difference is designated by .DELTA.x. When a gain of the position detector is denoted by K.sub.2, the deviation signal e is expressed by the following equation. EQU e=K.sub.2 .multidot..DELTA.x
The actuator outputs a reaction force F which encounters the external force F.sub.0, and the forces are balanced with each other so that the actuator becomes steady. Therefore, EQU F=F.sub.0
Further, providing that the gain of the amplifier is expressed by K.sub.1 and that the pressure gain K.sub.3 of the valve is linear and is not saturated, the output F is expressed by the following equation. In this equation, symbol A stands for an effective area of the actuator. EQU F=e.multidot.K.sub.1 .multidot.K.sub.3 .multidot.A
The ratio of a steady state deviation of a servo actuator, i.e., the difference between a desired value of a servo system and an actual position of an actuator rod, to an external force F will be referred to as rigidity K hereinbelow.
Accordingly, rigidity K is expressed as follows. ##EQU1##
When an electro-hydraulic servo actuator is used, for example, as an arm of an industrial robot or to control chucking of parts, to grip relatively soft materials the rigidity K must be lowered so as to prevent the objective materials from being damaged by an excessive load caused by deviation of control.
Contrary to this, the rigidity K must be increased during positioning control in order to decrease the steady state deviation caused by external force.
However, as shown by equation (1) above, in the conventional electro-hydraulic servo actuator, the rigidity K is indiscriminately determined based on the gain K.sub.1 of the amplifier, the gain K.sub.2 of the position detector, and the pressure gain K.sub.3 of the valve.
Accordingly, an electro-hydraulic servo actuator has been desired so that the rigidity can be readily set regardless of the gain K.sub.1 of the amplifier, the gain K.sub.2 of the position detector, and the pressure gain K.sub.3 of the valve.